1. Field of the Invention
The present invention relates to a method and an apparatus for compensation of any variation that occurs in the output power or luminous energy of an LED (light emitting diode) array in an LED array head used for writing in an electrophotographic optical printer.
2. Description of the Prior Art
With the recent enhancement attained in the processing capabilities of OA (office automation) apparatus, it is conspicuous that the demand for high-resolution fast recording apparatus such as optical printer is remarkably increasing. In particular, studies are in rapid progress with regard to an optical writing system employing a full solid-state head with an LED array, liquid-crystal shutter array or the like, so as to realize an extremely compact, lightweight and low-cost structure. Although such apparatus has some merits including adaptability to mass production, high reliability and facility for maintenance, there also exists a problem peculiar to an array light source that individual elements thereof are not free from performance variation.
Generally in an LED array head, there is adopted a means of covering a print area by arranging a multiplicity of monolithic LED array chips each composed of integrated 64 to 256 elements. And in the light source for an optical printer, the output power or luminous energy of each light emitting element needs to be substantially the same.
In the present technical stage, however, it is unavoidable that some performance variation is induced among the elements due to the defects in commercially available wafers or because of nonuniformity in the process of production. And such output power variation in the LED array eventually brings about nonuniform diameters of printed dots. It is therefore essential to compensate such variation in the optical writing system using an LED array head.
With regard to compensation of LED-array output power variation, the following two methods have been known heretofore.
(1) A technique of connecting series resistors of mutually different resistance values to the individual elements respectively and driving them at a constant voltage. (Compensation by hardware) PA1 (2) A technique of controlling a current application on-time for each of the elements so that the exposure values thereof become uniform on a photosensitive medium. (Compensation by software) PA1 T.sub.0 : common current on-time PA1 n: number of compensation steps PA1 t: fundamental on-time unit for compensation PA1 k: =0, 1, 2, . . . , n
According to the former method, the circuit configuration needs to be such that currents flowing in the individual elements of different luminous energies are changeable. And another disadvantage is that once the circuit is set, it is not alterable with facility to consequently cause considerable labor and increased cost in production.
Therefore, the latter method carried out with current on-time control is adopted more frequently. In general, an LED array head employed in the latter has a driving controller comprising a current on-time control circuit, an allotter circuit and a driving circuit. The on-time control circuit and the allotter circuit are formed separately from the head and are connected thereto by means of flat cables. Meanwhile the driving circuit is incorporated in the head and is formed into an IC configuration.
The current on-time control circuit serves to convert input serial data into parallel data and has an on-time compensational data memory ROM for compensating any variation in the LED-array output power. In the ROM is stored current on-time data representing the individual luminous intensities with respect to the entire bits of the LED array. Since such on-time data is determined from the luminous intensity of each bit obtained by actual measurement after complete assembly of the LED array head, composite compensation is performed inclusive of the nonuniformity of light transmitting elements as well. The allotter circuit serves to distribute the data to the integrated driving circuit related to the individual LEDs.
There are known two modes for executing such current on-time control. In the first mode, a one-line scanning time T is uniformly divided by the number m of steps, and a current is applied to each of the LED array elements for an on-time which is equal to an integral multiple of one minimum on-time unit .DELTA.t of each step thus divided. That is, the current on-time for any LED array element producing a smaller output power is changed to be longer than the on-time for the other LED array element producing a greater output power, whereby the luminous energies of the individual LED array elements are rendered uniform to consequently compensate the luminous energy variation.
Meanwhile in the second mode, a common current on-time T.sub.0 is set so that the entire LED array elements in one line are energized to emit light. And the time obtained by subtracting the common on-time T.sub.0 from the one-line scanning time T is uniformly divided by the number n of steps. And the luminous energy variation is compensated by the application of a current to any LED array element, which is producing a smaller output power, for a time equal to an integral multiple of the divided fundamental on-time unit t.
The current on-time .tau. determined for each element according to the compensational data is expressed as EQU .tau.(k)=T.sub.0 +(n-k)t
where
However, if the numbers m and n of steps are small in the current on-time control mentioned above, it is impossible to attain sufficient compensation effects for variation in the LED array elemenets; while if such numbers of steps are great, the compensational data come to increase with another problem that the composite luminous energy or output power of the entire elements is reduced by such compensation.